That rear suspension pictured looks really good!
I looked up the rear springs and from what I found all the Trans Am's in 79-81 had the same rear springs WS6 or not, though non-Trans Am's may have had something different. I don't think you can tell what a spring rate is just by counting the number of leafs. Eaton's web site has something that explains all the factors including thickness, diameter, etc...
If you knew the hassle with stock rear disc brakes and getting them adjusted you may be glad you have the rear drum. I have rear disc and I'm dreading the day I start working on them, which is going to be in the coming weeks. Do a search on this forum on the topic, you may be better off with one of those S10 or LS1 brake swaps, especially if you didn't plan on running an emergency brake.